In recent years, with intensive downsizing of semiconductor devices, a lithography using extreme ultraviolet (hereafter, referred to as “EUV”) light having a shorter wavelength has been considered in place of conventional ArF light. In the lithography using the EUV light, a mask is used under a certain degree of vacuum, and floating impurities are activated by the EUV light and may be attached onto the mask surface as contamination (dirt). When the contamination is attached onto the mask surface, the exposure amount is reduced and the dimension resulted after the exposure process may vary. Thus, a process is provided for cleaning up the mask that has been used for a certain time period to detach the contamination. Under the circumstances, however, the time until when the mask can be used is not known before the dimension actually changes, causing a problem.